Blastocysts

A blastocyst is a mammalian embryo at a late pre-implantation stage, forming about five to six days after fertilization in humans. It is characterized by a fluid-filled cavity called the blastocoel, an outer layer of cells called the trophoblast that contributes to the placenta and fetal membranes, and an inner cell mass that will develop into the embryo proper.

During development, the zygote multiplies to form a morula and then differentiates into the blastocyst. The trophoblast encases the blastocyst and mediates adhesion to the uterine lining, while the inner cell mass remains clustered inside. Implantation into the maternal endometrium typically begins around day 6 to 7 post-fertilization in humans and continues over several days, establishing the embryo’s early connection to the mother.

In humans and many other mammals, blastocyst-stage embryos are commonly used in assisted reproductive techniques. Blastocysts are often selected for transfer to improve implantation rates, and trophectoderm cells from a biopsy may be analyzed for preimplantation genetic testing. The inner cell mass can be a source for embryonic stem cell research, though such work raises ethical considerations in many jurisdictions.

Size-wise, a mammalian blastocyst is roughly 0.1 to 0.2 millimeters in diameter. While the blastocyst stage is a key benchmark in mammalian development, the exact timing and morphology can vary among species.